Charirs are known (for instance from DE-A-30 36 993) which can be used in particular as office swivel chairs and have a so-called "synchronising mechanism". These chairs react automatically or after actuation of a release means in such a way that when the user leans back the seat is lowered slightly and the back is inclined more sharply backwards, whilst the front edge of the seat remains approximately the same distance from the floor. This movement of the resulting opening of the angle between the seat and the back rest approximates the natural course of the movement of the human body. If the user again moves out of the leaned-back position into an upright working position, then the chair automatically follows by means of a spring mechanism fixed below the seat. Such changing of position facilitates adaptation to different activities at the workplace and are regarded by ergonomists and workplace doctors as preventive measures against the consequences to health of sedentary office work.
A chair disclosed in German application No. P 37 37 491.5 has a front lever connected via two swivel joints to a pedestal and the seat, whilst a rear lever is also connected via two further swivel joints to the pedestal and the seat, this rear lever having an extension which bears the back rest and projects over the fourth swivel joint. In this construction of the chair relative positions of the plane of connection between the first and second swivel joints with respect to the vertical on the one hand and of the plane of connection between the third and fourth swivel joints with respect to the vertical on the other hand should ensure favourable seat positions and changes of position.
Furthermore, a chair construction is also known from EP-A-250 207 in which at least one rear lever is connected on the one hand via a first swivel joint to the pedestal and on the other hand via a second swivel joint to the rear end of the seat, whilst at least one front lever is connected via a third swivel joint to the pedestal and via a fourth swivel joint to the front end of the seat. A spring system is arranged between the central column of the pedestal and the fourth swivel joint at the front end of the seat.
The object of the invention is to make further improvements to a chair of the type referred to and in such a way that it is adapted particularly easily and well to the natural movement of a person at a desk and is distinguished above all by a relatively simple design.